The aerospace industry has unique requirements for the insulation of wires and cables. Due to weight constraints, the industry has progressively moved toward thinner insulation for wires and wire bundles. The extrusion of melt processable fluoropolymers is well known in the aerospace field for manufacturing an insulation layer for high temperature wire and cable. Because the melt fluoropolymer materials can melt and fuse together, the material is laid down in one continuous layer. The melt fluoropolymer extrusion process provides a layer of insulation with a generally smooth exterior surface around the wire, without any seam or overlap, and therefore the extrusion construction is generally considered “seamless.” However, the extrusion of melt fluoropolymer films has not been as reliable as desired with respect to obtaining a uniform, thin layer of insulation on the wires. This drawback has led to the use of unsintered polytetrafluoroethylene (PTFE) and tape wrap for insulating wires and cables. The tape insures a layer of relatively uniform thickness for insulating and protecting the wire. However, by wrapping the tape around the wire, and then attempting to fuse the layers of tape, an overlap seam is created, as shown typically in FIG. 3. This type of seam has been a cause of significant concern. For example, the overlapping seam can be a significant drawback when the wires are installed into the frames of airplanes or other aircraft. Such wires are required to pass a variety of relatively extreme tests, including a wet arc electrical insulation test, a scrape abrasion test, and a minimum contrast level for marking wires test. The PTFE tape wrapped constructions have had difficulty consistently meeting the criteria for, or in passing all of these tests.
Many prior art tape wrapped constructions are supplied using a tape that is known as an unsintered extruded PTFE, which is extruded in a flat form, calendared to a desired thickness, and then slit to a desired width. Specialized equipment is then used to wrap the tape onto wires or bundles of wires, and then the wrapped wires or bundles of wires are heated (or sintered) to fuse the wrapped PTFE tapes thereto. The PTFE tapes typically include fillers that turn the tape white when wrapped and sintered. This material forms the outer layer of insulation for the wires or the outer layer of the wrap or jacket for bundles of wires. The PTFE tape can be filled with a material, such as TIO2 or a similar filler that is white, but that turns dark (e.g., black) when a laser of known energy and/or wavelength is transmitted into the filler within the PTFE tape. As a result, the laser can be used to mark indicia, such as numbers, letters or others markings to identify, for example, a specific wire at the ends of long lengths of wires or cables. One of the drawbacks of these fillers is that it can be difficult to disperse them evenly or uniformly within the PTFE, and as a result, it can be difficult to uniformly or clearly mark the wires, or uniformly mark the wires throughout the lengths of wire. When the filler materials are not dispersed evenly, the marking will vary, and will not meet the uniform contrast levels that must be achieved or that otherwise are desired. Another and even more alarming drawback associated with non-uniform distribution of the filler material is that it can form agglomerations which can, in turn, cause voids in the insulation that can lead to shorting of the wire. The testing of these wires can include a “wet arc” test which is an aggressive aerospace test that checks the finished wire in a wet environment to find weak spots in the wires. Tape wrapped wires with fillers in the PTFE can have difficulty in passing such tests, or can lead to shorting of the wires if the problems are not detected during testing.
Another drawback of the above-mentioned PTFE tape wrap is the overlapping seam. The overlapping seam creates a ridge at the overlap as shown typically in FIG. 3. The PTFE does not undergo a melt process as does the melt processable fluoropolymers described above, but rather goes into a transition or gel-like state when heated. When in the transition state, the PTFE may bond to itself and form a seal at the overlapping seam. However, the inherent properties of PTFE can prevent the formation of an adequate bond or seal. One of the tests performed on such wrapped wires to ensure that the overlap is sufficiently bonded is a “scrape abrasion test.” In this test, a bar is brought into contact with the insulated wire, is pressed against the wire with a predetermined force, and is moved or rubbed along a length of the wire at that force, as shown typically in FIGS. 4A and 4B. In order to pass the test, the finished wire must last a defined number of cycles of rubbing without abrading or separating the layers of PTFE. This test has given rise to significant problems with respect to separating layers of tape in PTFE wrap, and many solutions have been reviewed to try to solve the problem. One approach has been referred to as a “seamless tape” because, even though the wire is tape wrapped, the exterior surface resembles that of extrusion made wire. This type of wire uses a tapered-edge film. As shown typically in FIG. 6, the tapered-edge films are overlapped such that the overlap area has very little height differential due to the tapered edge, and thus the finished product can look as though it does not have an overlap or seam. As a result, there is less overlap material to present an edge that can be caught during the scrape abrasion test. This approach has had improved success with respect to scrape abrasion testing. However, since there is still a seam of some dimension, and because of the particular characteristics of the overlapping PTFE fused to itself at the seam, there nevertheless can be problems with abrading and separation at the tapered-edge seams when subjected to the scrape abrasion test.
It is an object of the present invention to overcome one or more of the above-described drawbacks and/or disadvantages of the prior art.